Xenon external recycling unit for recovery, purification and reuse of xenon in anaesthesia circuits

ABSTRACT

The present invention concerns the application of adsorption gas separation technologies to anaesthesia equipment and falls within the technical domains of adsorption separation units and medical devices. The invention concerns a device and processes for recovering xenon from gas mixtures released from anaesthesia gas machines ( 1 ) using xenon as anaesthetic. The purged xenon is collected using a system which includes a shift valve ( 6 ), and then separated and purified ( 38 - 40 ). The recycled xenon is then pressurised ( 31 ) and reintroduced in the anaesthesia circuit ( 48 ) using a shift valve ( 12 ). The separation and purification process combines different adsorption separation/purification technologies. The device is external to the anaesthesia gas machine and is compatible with any standard anaesthesia circuit able of perform xenon anaesthesia.

This application is a National Stage Application of InternationalApplication Number PCT/EP03/04568, filed Apr. 30, 2003; which claimspriority to Portugal Application No. PT 102772, filed May 2, 2002.

FIELD OF THE INVENTION

The present invention concerns the application of adsorption gasseparation technologies to anaesthesia equipment and falls within thetechnical domains of adsorption separation units and medical devices.The invention concerns a device and processes for selectively recoveringxenon from gas mixtures released from anaesthesia gas machines (1) usingxenon as anaesthetic. The purged xenon is collected, purified and thenconditioned for reuse. The device is external to the anaesthesia gasmachine and is compatible with any standard anaesthesia circuit able ofperform xenon anaesthesia.

BACKGROUND OF THE INVENTION

Xenon is a very efficient anaesthetic gas, however it is an expensiveand a rare resource. Recycling can significantly reduce the high costsassociated with its use as anaesthetic. Two different xenon-recyclingapproaches can be taken: internal recycling (performed inside theAnaesthesia Gas Machine—AGM) and external recycling (performed outsidethe Anaesthesia Gas Machine—AGM). Two solutions have been proposed forinternal recycling (bibliography [1] and [2]): the first proposed byUniversity of Porto (Faculty of Engineering—FEUP), considers theconstant removal of carbon dioxide and nitrogen from a closed circuitAGM, using adsorption and/or membrane technology; the second, proposedby Nippon Oxygen Corporation, considers only the removal of nitrogen,consequently reducing the AGM purge waste.

Two different kind of technological solutions have been proposed forxenon external recycling:

-   -   High-pressures and/or cryogenic technologies—bibliography [3] to        [7].    -   Adsorption based technologies—bibliography [8] and [9].

The first kind requires the manipulation of pressure and/or temperaturein order to condensate xenon from the anaesthetic gas mixture. Thisprocess is used thoroughly in xenon industrial separation from air,however, the high volume, noise and power consumption of the requiredequipment make this process less attractive for smaller scales.

The second kind requires the use of specific adsorbents in order toselectively adsorb some of the components of the anaesthetic gasmixture. After treatment, the recycled xenon is usually pressurized in agas cylinder.

SUMMARY OF THE INVENTION

Anaesthesia Gas Machines (AGMs) are used whenever it is necessary toperform gaseous anaesthesia. These devices deliver a precisely-known butvariable-gas mixture, including anaesthetizing and life-sustaininggases.

Usually two types of AGMs are used for xenon anaesthesia: closed-loopAGMs and low-flow AGMs.

In closed-loop AGMs, the anaesthetic gas mixture circles in aclosed-loop circuit being the gas mixture continuously fed to thepatient. During the breathing process the patient inhales oxygenexhaling carbon dioxide and nitrogen. Carbon dioxide is continuouslyremoved from the anaesthetic circuit by absorption containers (usuallycontaining soda lime). The required oxygen concentration is continuouslyrestored by the control system of the anaesthetic circuit. The nitrogencontinuously exhaled by the patient builds-up in the closed loop. Whenits concentration reaches approximately 5% it becomes a problem and thewhole anaesthetic gas mixture is purged and a new mixture is supplied.Usually the purged anaesthesia gas mixture is disposed of outside theOperation Room (OR) through an external ventilation system (Europeanstandards DIN13260 or EN740). Frequently, pure oxygen is previouslygiven to the patient in order to remove the nitrogen in his body, thisremoval is not complete and therefore the referred purges are necessary.

In, low flow AGMs the flow of anaesthetic gas mixture supplied to thepatient is slightly above his breathing needs. Exhaled and excess flowsare purged from the anaesthetic circuit. Carbon dioxide can be totallyremoved using an absorption system. The purged gas mixture is usuallydisposed of through an exterior ventilation system.

In an average surgery approximately 13.5 litres of xenon are spent. Thepurpose of the present invention is the recovering and recycling thexenon released from the anaesthetic circuits for being reused in thecircuit. The proposed Xenon External Recycling Unit (XERU) is completelyexternal and independent from the AGM, i.e. it does not interfere withthe anaesthetic gas mixture circulation during anaesthesia, nor with anyof the AGM functionality, either if the Xenon External Recycling Unit(XERU) is activated or not.

The sketch in FIG. 1, illustrates the proposed device and process, whichare based in the following stages:

-   -   The Selective Recovery System (SRS) of the anaesthetic gas        mixture released by the AGM;    -   a condensation removal system;    -   a gas sterilisation system;    -   a System of Purification (SP) of the xenon present in the stored        anaesthetic gas mixture;    -   a composition detector;    -   a Recycled Xenon Introduction System (RXIS).

The anaesthetic gas mixture is recovered through a gas line connectingthe Anaesthesia Gas Machine (AGM) vent relief valve to the XenonExternal Recycling Unit (XERU) (1). The purged anaesthetic gas mixture,containing xenon, is recovered instead of being disposed of through theexternal ventilation system (49). The recovery is performed by aSelective Recovery System (SRS).

The Selective Recovery System (SRS) comprises a flowmeter (either a massor a volumetric flowmeter), a composition detector and a 3-way valve.The 3-way valve (6) can be either activated or not; if activated thevalve connects the Anaesthesia Gas Machine (AGM) to the Xenon ExternalRecycling Unit (XERU), if not it connects the Anaesthesia Gas Machine(AGM) to the external ventilation system. By default, the valve is notactivated, when so, the Xenon External Recycling Unit (XERU) does notinterfere with the Anaesthesia Gas Machine (AGM) purge, this correspondsto the safer position.

If the Xenon External Recycling Unit (XERU) power supply is cut, if theXenon External Recycling Unit (XERU) maximum capacity oftreatment/storage of anaesthetic gas mixture is reached, if any abnormalsituation occurs or if the anaesthetist decides to, the 3-way valve isdeactivated and the anaesthetic gas mixture is directed to the externalventilation system. Under standard operation conditions, the valve isactivated if the gas stream released by the Anaesthesia Gas Machine(AGM) has a minimum flow and a minimum xenon composition.

During the surgery the xenon composition in the purged anaesthetic gasmixture fluctuates. The Selective Recovery System (SRS) allows narrowingthe xenon composition range of the anaesthetic gas mixture treated bythe System of Purification (SP).

The anaesthetic gas mixture collected by the Selective Recovery System(SRS) is directed through a filter system (13-14) which full removesbacteria and reduces all the condensable present in the anaesthetic gasmixture up to a molar concentration below 0.5%. The main condensable areusually water vapour, volatile organic compounds and high-powerfulanaesthetics (fluoranes). There are several commercially availableoptions for condensable removal: membrane filters, adsorption filters,driers, etc.

The filtered anaesthetic gas mixture, mainly composed of xenon, oxygenand nitrogen, is directed to a packed column (38), which is filled witha selective adsorbent, e.g. zeolite 5A, and has a hole in each end. Thefirst hole is for feeding the filtered anaesthetic gas mixture and thesecond is the exit. The adsorbent filled packed column can selectivelyretain high-quantities of xenon in a small volume at low pressures.

The adsorbent in packed column selectively absorbs more xenon thanoxygen and nitrogen. It was experimentally verified (FIG. 2) thatzeolite 5A adsorbs 5 times more xenon than nitrogen and 14 times morexenon than oxygen (at 1 atm).

The packed column must be previously evacuated before the anaestheticgas mixture is fed. The xenon contained in the fed anaesthetic gasmixture is selectively adsorbed in the packed column; the remaininggases leave column and are disposed of. When the capacity of theadsorbent is reached, the packed column stops retaining xenon whichleaves the column (FIG. 3). Before this happens, the column should beisolated to prevent xenon losses.

A 3-way valve (7) allows directing the gas leaving the packed columneither to be disposed of through an external ventilation system (49), ifthe xenon concentration is below a certain threshold or to apurification unit, otherwise. The xenon concentration in the gas streamleaving the column is analysed using a concentration detection system(4).

When the packed column was filled with 2 kg of zeolite 5A, it waspossible to treat 112 litres (STP) of anaesthetic gas mixture(equivalent to the amount of gas purged in 5 to 6 average surgeries)containing 70%-24%-6% of xenon-oxygen-nitrogen at a pressure of 1.4bara. It was possible to remove more than half of the oxygen-nitrogenpresent in the gas mixture with xenon losses below 5%.

Since the composition of the purged anaesthetic gas mixture is notconstant, a detection system at the exit of the packed column is used toprevent xenon losses. This detection system can either be a compositionsensor or a temperature sensor, placed near the column end, that detectsthe heat released during xenon adsorption.

When the packed column is filled to capacity, the comprised gas mixtureis removed and the xenon is purified. This is performed using a vacuumpump or/and a heating system.

Xenon is purified in an adsorption system using Vacuum Swing Adsorption(VSA), in which the adsorbent selectively retains all components butxenon, and the adsorbent is regenerated by total pressure decrease.Separation is achieved using a carbon molecular sieve (e.g. Takeda CMS,Japan), a kinetic adsorbent (i.e. based on the different diffusion timesof the gases in the adsorbent).

The gas stream to be treated has an average xenon composition of 70%,being the remaining components oxygen, nitrogen and residual amounts(below 0.5%) of fluoranes, water vapour and volatile organic compounds.Breakthrough experiments with Takeda's CMS using a gas mixturecontaining 70%-24%-6% of xenon-oxygen-nitrogen, showed a low retentiontime for xenon, indicating that this gas is not significantly adsorbed(FIG. 4); on the other hand, oxygen, nitrogen and carbon dioxide arestrongly adsorbed.

For the mentioned fed composition, the VSA unit can achieve a xenonpurity of 97% or above with a xenon recuperation above 70%. This isperformed using the following procedure.

The unit consists in a pair of packed columns filled with a selectiveadsorbent (e.g. Takeda's CMS) operating in a 180° out of phase cycle,comprising the following sequential steps:

-   -   a) a pressurisation step, lasting for 1 to 20 seconds, in which        the pressure inside the packed column is increased from a        sub-atmospheric pressure to a given pressure, first by using a        gas stream from the other column and then using the gas stream        to be recycled; the column end opposite to the feed end is kept        closed;    -   b) a production step, lasting less than 10 minutes, in which the        gas stream is fed to one end of the packed column, and a rich        xenon concentration stream is produced on the other end of the        column by retaining the non-xenon components in the adsorbent;        this step is finished when the adsorbent has no longer capacity        to retain the non-xenon fed components;    -   c) an equalisation step, in which the gas mixture, containing        xenon, present in the packed column inter-particular phase is        recovered in the second column, by connecting the two columns        and allowing the pressurised gas mixture to flow to the        evacuated second column until pressure in both columns is        partially or totally equalised;    -   d) an evacuation step, succeeding the equalisation step, in        which the packed column is regenerated by using a vacuum pump or        other device (42), preparing the packed column for the next        step;

A system of electro-valves (8, 9, 33, 34) and check valves (19, 20, 21and 22) allows to conveniently direct the gas stream.

The recycled xenon (with a concentration above 97%) is stored in astainless steel reservoir (41). This is achieved either by compressingthe gas, or by using a column containing a specific adsorbent (e.g.zeolite 5A). This last solution, allows storing a large amount of xenonin a small volume even at low pressures. A column containing twokilograms of zeolite 5A allowed storing 50 litres (STP) of xenon at 1bara, while the storing the same quantity in an empty column wouldrequire a pressure of 25 bara.

The recycled xenon is reused through the pressurised (8 bar) gas linethat feeds the pure xenon (usually in a gas cylinders or in a proper gasline) to the Anaesthesia Gas Machine (AGM). This is performed through a3-way electro-valve (10) which can be either activated or not; ifactivated the valve connects the container storing recycled xenon to theAnaesthesia Gas Machine (AGM), if not it connects gas cylindercontaining xenon to the Anaesthesia Gas Machine (AGM). By default, thevalve is not activated, when so, the Xenon External Recycling Unit(XERU) does not interfere with the Anaesthesia Gas Machine (AGM) supplyfrom the xenon gas cylinder, this constitutes the safer position.

If the Xenon External Recycling Unit (XERU) power supply is cut, if norecycled xenon is available in the External Recycling Unit (XERU), ifany abnormal situation occurs or if the anaesthetist decides to, the3-way valve is deactivated and the anaesthetic gas mixture is fed by thexenon gas cylinder. Under standard operation conditions, the valve isactivated if the recycled xenon stored inside the External RecyclingUnit (XERU) has a minimum pressure (45) and a minimum purity (5).

If an empty reservoir is used to store the recycled xenon, this can befed to the Anaesthesia Gas Machine (AGM) using a pressure regulator toset pressure at 8 bar. If an adsorbent filled reservoir is used instead,the stored gas must be previously pressurised at 8 bar (e.g. using acompressor) before fed to the Anaesthesia Gas Machine (AGM) through the3-way electro-valve.

DESCRIPTION OF THE DRAWINGS

In FIG. 1, a schematic diagram, is proposed as a non confining exampleof the presented invention. FIG. 1 shows:

-   1—Vent relief valve of the valve of the Anaesthesia Gas Machine    (AGM)-   2—Flowmeter-   3 to 5—Composition sensor-   6 to 12—3-way electro-valve-   13—Condensation removal system-   14—Gas sterilisation system-   15 to 28—Check valve-   29 to 31—Compressor-   32 to 35—Electro-valve-   36, 37—Pressure regulator-   38 to 40—Packed column-   41—Packed column with heater-   42—Vacuum pump-   43, 44—Needle valve-   45—Pressure sensor-   46—Membrane system-   47—Input gas line connecting to the xenon gas cylinder-   48—Output gas line connecting to the Anaesthesia Gas Machine (AGM)

FIGS. 2, 3 and 4, show:

FIG. 2 shows the adsorbed amount of xenon, oxygen and nitrogen inzeolite 5A as a function of the pure gas pressure. The experience wasperformed using the volumetric method at 20° C.±1° C., with a mass ofadsorbent of 28.9 g; the holding tank volume was 202.8±0.6 cm³, thesample tank volume was 45.4±0.2 cm³ and the adsorbent volume was12.3±0.2 cm³.

FIG. 3 shows the xenon, oxygen and nitrogen composition of a gas streamleaving a 100 cm³ packed column filled with zeolite 5A, as function oftime. The packed column initially evacuated was pressurised and fed witha gas stream containing 70%-24%-6% of xenon-oxygen-nitrogen at adecreasing pressure (from 1.9 bara to 1.4 bara) and a decreasingflowrate (from 120 to 60 cm³/min.)

FIG. 4 shows the xenon, oxygen and nitrogen composition of a gas streamleaving a 1725 cm³ packed column filled with CMS, as function of time.The packed column initially evacuated was pressurised and fed with a gasstream containing 70%-24%-6% of xenon-oxygen-nitrogen at a constantpressure (1.4 bara) and a decreasing flowrate (from 120 to 60 cm³/min.)

BIBLIOGRAPHY

-   [1]—MENDES, ADÉLIO; “DISPOSITIVO E PROCESSO DE REMO    ĀO DO DIÓXIDO DE CARBONO E DO AZOTO DE UM CIRCUITO ANESTÉSICO    FECHADO USANDO XÉNON”; PT102416, 2001.-   [2]—HAYAKAWA, SABURO; TAKEUCHI; NAOKO; NAKAMURA, AKIHIRO; “METHOD    AND DEVICE FOR ANESTHESIA USING XENON”; JP2001252358, 2001.-   [3]—GEORGIEFF, MICHAEL; MARX, THOMAS; BADER, STEFAN; “ANESTHESIA    ARRANGEMENT FOR RECOVERING GASEOUS ANESTHETIC AGENTS”; DE5520169;    1996.-   [4]—BUROV, N. E.; MAKEEV, G. N.; “METHOD AND DEVICE FOR RECLAIMING    XENON FROM NARCOTIC GAS MIXTURE IN ANESTHESIA APPARATUS”; RU204948,    1995-   [5]—ESCHWEY, MANFRED; HAMM, REINER; NEU, PETER; SCHMIDT, RENATE;    SCHROEDER, GEORG; “ON-LINE RECOVERY OF XENON FROM ANAESTHETIC GAS”;    WO9808583, 1998.-   [6]—BOSO, LUCA; “PROCESS AND APPARATUS FOR PURIFYING AND RECOVERING    XENON AND OTHER NOBLE GASES USED IN ANAESTHETIC SYSTEMS”; WO    9818718, 1998.-   [7]—HAMM, REINER; “SEPARATION OF XENON FROM GAS MIXTURES OR REMOVAL    OF HIGH PURITY XENON FROM ANAESTHETIC GAS—USING PROCESS HAVING TWO    OR MORE SEPARATION STAGES”; EP901985, 1999-   [8]—BUROV, N. E.; POTAPOV, V. N.; EFIMOV, V. V.; MAKEEV, G. N.;    SURNIN, A. G.; VOVK, S. M.; “METHOD AND DEVICE FOR REGENERATING    XENON FROM NARCOTIC GAS MIXTURE USED IN ANESTHESIA APPARATUS”;    RU2149033; 2000.-   [9]—DRAEGER AEROSPACE GMBH; “SELECTIVE ADSORPTION APPARATUS FOR    REMOVING XENON FROM MIXTURE WITH NITROGEN AND OXYGEN, USEFUL IN    ANAESTHETIC—USES COMMERCIALLY-AVAILABLE TYPE X, ZSM-5 OR A ZEOLITE    OF SPECIFIED GROUP AND PORE SIZE IN COMPACT UNIT WITH FEW    COMPONENTS”; DE29817824, 1998.

1. A Xenon External Recycling Unit (XERU), for recycling xenon from agas mixture released by an Anaesthesia Gas Machine (AGM) using xenon asanaesthetic gas, comprising: a) a Selective Recovery System (SRS) thatis connected to a vent relief valve of the AGM through a gas line thatis able to collect the released gas mixture, provided that it has aminimum flow, a minimum xenon composition, and a pressure variationbelow a threshold value; wherein the SRS comprises: a shifting systemfor directing the gas mixture either to a storage or conditioningsystem, if said gas mixture contains a minimum threshold of xenonconcentration, or otherwise to an output line; and a safety system thatcan override the normal operation of said Selective Recovery System(SRS) purging the gas mixture to the outside; b) a condensation removalsystem; c) a gas sterilization system; d) an adsorption-based System ofPurification (SP), wherein said SP comprises at least one stage able toseparate the xenon in said gas mixture with a selective adsorbent; e) acomposition detector for analyzing the recycled xenon; and f) a RecycledXenon Introduction System (RXIS) that feeds the Anaesthesia Gas Machine(AGM), provided that the recycled xenon has a minimum xenon composition.2. The XERU, according to claim 1, wherein the output line is selectedfrom the group consisting of: a) a line connecting the gas to theoutside; b) a line connecting the gas to an outside ventilation system;c) a line connecting the gas to a scavenger system; and d) a lineconnecting the gas to a recycling unit.
 3. The XERU, according to claim1, wherein said safety system is triggered by any one or more of thegroup consisting of: a) the maximum capacity of treatment/storage of thegas mixture is reached; b) the Xenon External Recycling Unit (XERU) isnot active; c) the pressure variation in the gas line connecting theAnaesthesia Gas Machine (AGM) to the Selective Recovery System (SRS) isabove a certain threshold; and d) a manual command is given.
 4. TheXERU, according to claim 1, wherein said System of Purification (SP)comprises: a) a packed column containing an adsorbent that selectivelyadsorbs xenon over oxygen and nitrogen; b) a mechanism for driving thegas mixture through the packed column; c) a system for detectingcomposition variations in the gas stream; and d) a heating device,associated with the packed column, to facilitate the removal of adsorbedxenon.
 5. The XERU of claim 4, wherein the selective adsorbent includedin the packed column is zeolite 5A or Li-X.
 6. The XERU of claim 4,wherein the non-adsorbed gas stream, with a low xenon concentration, isdirected to the atmosphere or to a device that disposes of thenon-adsorbed stream.
 7. The XERU, according to claim 1, wherein saidSystem of Purification comprises a pair of packed columns, in anarrangement known as “Vacuum Pressure Swing Absorption”, wherein: a) thepacked columns contain an adsorbent for performing the kineticseparation of the xenon from oxygen and nitrogen, by not adsorbingxenon; b) the packed columns operate in a 180° out of phase cycle; c)each cycle includes the stages: pressurization, production,equalization, and evacuation; and d) control is achieved using anautomatic system, such as a logical control unit and electro-valves. 8.The XERU, according to claim 7, the use of which comprises the followingfour steps in a cyclic sequence: a) a pressurization step, lasting for 1to 20 seconds, in which the pressure inside the packed column isincreased from a sub-atmospheric pressure to a given pressure, first byusing a gas stream from the other column and then using the gas streamto be recycled, keeping closed the column end opposite to the feed end;b) a production step, lasting less than 10 minutes, in which the gasstream is fed to one end of the packed column and a rich xenonconcentration stream is produced on the other end of the column byretaining the non-xenon components in the adsorbent, finishing this stepwhen the adsorbent has no more capacity to retain the non-xenon fedcomponents; c) an equalization step, in which the gas mixture,containing xenon, present in the packed column inter-particular phase isrecovered in the second column, by connecting the two columns andallowing the pressurized gas mixture to flow to the evacuated secondcolumn until pressure in both columns is partially or totally equalized;and d) an evacuation step, succeeding the equalization step, in whichthe packed column is regenerated by using a vacuum pump or other device,preparing the packed column for the next step.
 9. The XERU according toclaim 7, wherein the adsorbent included in the packed columns is aCarbon Molecular Sieve.
 10. The XERU according to claim 7, wherein thegas stream produced during the evacuation stage, is directed to at leastone of the group consisting of: a) a line connecting the gas stream tothe outside; b) a line connecting the gas stream to an outsideventilation system; c) a line connecting the gas stream to a scavengersystem; and d) a line connecting the gas stream to a recycling unit. 11.The Xenon External Recycling Unit (XERU) of claim 1, wherein said Systemof Purification comprises an arrangement of two or more purificationstages.
 12. The XERU according to claim 1, wherein the RXIS comprises:a) means of connecting the Recycled Xenon Introduction System (RXIS) toa xenon input channel of the Anaesthesia Gas Machine (AGM); b) aselection system for directing to the Anaesthesia Gas Machine (AGM)either the recycled xenon, if its purity is above a minimum threshold,or xenon supplied from an external source, otherwise, and c) a safetysystem which can override the normal operation of said Recycled XenonIntroduction System (RXIS), thereby interrupting the feeding of recycledxenon, allowing only the pure xenon from an external source to be fed tothe Anaesthesia Gas Machine (AGM).
 13. The XERU according to claim 12,wherein said xenon input channel is an anaesthetic gas feed valve. 14.The XERU according to claim 12, wherein said external source is a gascylinder with pure xenon or a gas mixture containing xenon.
 15. TheXERU, according to claim 12, wherein the RXIS comprises a safety system,triggered by any one or more of the conditions selected from the groupconsisting of: a) the available amount of recycled xenon, or gas mixturecontaining xenon, is below a certain threshold; b) said Xenon ExternalRecycling Unit (XERU) is not active; c) the pressure variation in thegas line connecting said Anaesthesia Gas Machine (AGM) to said RecycledXenon Introduction System (RXIS) is above a certain threshold; and d) amanual command is given.